This research will examine the effects of dynamic loading and fatigue on the lower back by utilizing models of muscle response, body motion, and disc compression. Current models for predicting disc compression are, generally, static models. These models accommodate dynamic lifting by measuring hand forces during a dynamic lift and using the values at several static postures throughout the lift corresponding to those positions in time. In these studies subjects are told to lift slowly, that is, lift with very low accelerations. Unfortunately, this is not how most manual material handling is performed. Other back injuries include those arising from successful slip or other loss of balance recoveries. Regaining postural control after a loss of balance is often difficult, requiring the quick application of high forces. These dynamic forces can overwhelm the body's natural injury defense mechanisms. In these studies subjects will be instructed to maintain a balanced, upright posture. The Torso Motion Measurement System (ToMMS, under current development) will then transmit an impulse to the pre-loaded (three levels) subject and their movement and muscle response will be recorded. The data will be analyzed and compared to current bio-mechanical models. Muscle fatigue will be induced through a series of graded exercises for three levels of fatigue (rested, moderate fatigue, tired). Torso muscle response will be determined and compared to control strategies for a dynamic torso model. Finally, analyses of pre-load and muscle fatigue response effects on compressive forces, control strategies, and muscle response parameters will be performed. It is stated that knowing the musculoskeletal response as a function of a person's loading and level of fatigue may enable researchers and clinicians to better examine and treat some of the basic causes of low back pain.